FIG. 8 is a view showing a conventional plastic molding device for plastic molding a semiconductor element.
In FIG. 8, reference numeral 80 designates a plastic molding means for plastic molding a semiconductor element, which comprises separable upper mold 70 and lower mold 71. The upper and lower molds 70 and 71 are supported by upper and lower platens 65 and 64 of a clamping press (a mold driving mechanism) which is not shown, respectively.
The upper mold 70 comprises an upper mold cavity block 1 having a plurality of upper mold cavities 1a, an upper surface plate 5 with a heater built therein which supports the upper block 1, a base plate 9 supporting the upper surface plate 5 through a post 15, and a spacer block 8 interposed between the upper surface plate 5 and the base plate 9. The upper cavity block 1 has a center block 2 in the center thereof and the center block 2 is equipped with a chamber 3 for housing a plastic tablet 14. At an upper portion of the chamber 3 a plunger 12 is provided for pressing the plastic tablet 14. In addition, an eject pin 4, one end of which is drawn in or out of the upper cavity 1a is provided in the cavity block 1 and the upper surface plate 5. Vertically movable pin supporting member 20 comprising an ejector plate 7 and a keep plate 6 which pushes the eject pin 4 toward the side of the ejector plate 7 is provided between the upper surface plate 5 and the base plate 9 in the upper mold 70. The pin supporting member 20 is urged against the upper surface plate 5 by forcing means 21. The forcing means 21 comprises a screw bar 10 screwed on the upper surface plate 5 and a spring 11 provided between the screw bar 10 and the pin supporting member 20. Reference numeral 13 designates a return pin fixed on the pin supporting member 20.
The lower mold 71 has the almost same structure as that of the upper mold 70, which comprises a lower mold cavity block 51 having a plurality of lower mold cavities 51a, a lower center block 52 at the center thereof, a lower surface plate 55 with a heater built therein which supports the blocks 51 and 52, a post 63 supporting the lower surface plate 55, a spacer block 58, and a base plate 59. Eject pins 54, one end of each of which can be drawn in and out of the lower mold cavity 51a, are provided in the cavity block 51 and the lower surface plate 55. A vertically movable pin supporting member 30 comprising an ejector plate 57 and a keep plate 56 is provided between the lower surface plate 55 and the base plate 59. The pin supporting member 30 is urged away from the lower surface plate 55 by a spring 60.
In the lower center block 52 and the lower mold cavity block 51 runners 52a and 51b are respectively formed as passages for the plastic 14. In addition, in the block 51 a gate 51c connecting the runner 51b to the lower mold cavity 51a is formed. The runner 52a and the gate 51c form a plastic injecting mechanism with the plunger 12 and the chamber 3. In addition, a return stopper 62 is attached to the lower surface plate 55. This return stopper 62 abuts the return pin 13 at the time of clamping to retract the eject pin 4 of the upper mold 70. A knockout rod 61 is fixed on the base part of the device. This knockout rod 61 abuts the pin supporting member 30 to eject the eject pin 54 into the lower mold cavity 51a when the upper and lower molds 70 and 71 are separated by moving the lower platen 64 downward. Further, a plurality of posts are provided in this device other than the above posts 15 and 63.
Next, the operation of the apparatus of FIG. 8 will be described.
A lead frame on which semiconductor chips are bonded is set on the lower cavity block 51. These chips are connected to the lead frame by metal wires. Then, the lower mold 71 is moved upward by the clamping press and put together with the upper mold 70 and then they are clamped. At this time, since the return stopper 62 pushes the return pin 13, the pin supporting member 20 slightly moves upward against the force of the spring 11, whereby the eject pins 54 are retracted from the upper mold cavity 1a.
Then, the plastic 14 which is preheated is put in the chamber 3 and the plunger 12 is moved downward to apply pressure to the plastic 14. Then, the plastic 14 passes through the runners 52a and 51b and then the gate 51c, and fills the cavities 1a and 51a.
In this state, the plastic is left for 60 to 90 seconds to solidify and then the lower mold 71 is moved down to open the plastic molding means 80. At this time, the return stopper 62 is separated from the return pin 13, whereby the pin supporting member 20 is moved downward by the force of the spring 11. Then, the upper mold eject pins 4 push plastic molded products out from the upper mold cavities 1a. Thereafter, the lower platen 64 of the clamping press falls and the knockout rod 61 abuts the bottom of the supporting member 30. Then, the pin supporting member 30 is pushed by the rod 61 and rises relative to the lower surface plate 55. Then, the eject pins 54 project into the lower cavities 51a and push the products out. The plastic molded products are picked up and then go through the process of cutting and bending the leads, completing plastic molded semiconductor devices.
In the conventional plastic molding device constituted as described above, the plastic is injected by the plastic injecting mechanism, but sufficient injection pressure does not reach the cavity because of the pressure loss in the runners 52a and 51b and the gate 51c or the like. Further, since the sectional area of the gate 51c is small as compared with that of other passages such as the runner, the plastics are likely to thermally harden there. When the plastic solidifies in this portion, the injection pressure does not reach the cavity at all. Accordingly, shrinkage deformation or the like occurs on the surface of the plastic molded product molded in the cavity causing the quality of the package to degrade. Particularly, in an optical semiconductor element for processing an optical signal, which is molded in transparent plastic, signal light reflects or refracts because of unevenness of the package surface, causing erroneous operation.
In order to solve the problem of shrinkage deformation in general plastic molding, there is proposed a device disclosed in Japanese Utility Model Laid-Open Application No. 63-191013, in which a cylinder and a piston are arranged corresponding to a thick portion of the molded product and pressure is applied to the plastic by actuating the piston after the plastic is injected. However, since there are many small cavities in the plastic molding device for a semiconductor element, it is actually impossible to arrange a cylinder and piston for each cavity, so that the problem of shrinkage deformation in plastic molding for the semiconductor element can not be solved.